PB1741: DIFFERENTIAL DIAGNOSIS OF T-LYMPHOBLASTIC LYMPHOMA/LEUKEMIA AND THYMOMA USING FLOW CYTOMETRY IN PLEURAL FLUID
Authors: Goeun Bae, Hee-Jin Kim, Sun-Hee Kim, Duck Cho, Hyun-Young Kim
Journal: HemaSphere (2023)

Topic: 2. Acute lymphoblastic leukemia - Clinical Background: Multiparametric flow cytometry (MFC) for body fluids is useful for rapid and less invasive diagnosis of suspected hematological malignancies in body fluids. However, it requires in-depth interpretation of immunophenotypic characteristics in consideration of normal or reactive cells present in body fluids. Thymocytes could be observed in the pleural fluid under clinical conditions including chylothorax and thymoma, and distinguishing T-lymphoblastic leukemia/lymphoma (T-LBL) from normal thymocytes in the pleural fluid is challenging. Aims: We studied immunophenotypic characteristics to differentiate between T-LBL and normal thymocytes originated from thymoma in the pleural fluid using MFC. Methods: This study included patients with morphologically immature lymphoid cells in the pleural fluid and anterior mediastinal mass. The pleural fluid was collected by thoracentesis for all patients and MFC was performed using two eight-colored tubes including CD45, CD3, cytoplasmic CD3 (cCD3), CD34, nuclear TdT (nTdT), CD4, CD8, CD1a, CD2, CD5, CD7, and CD10. For definite diagnosis, immunohistochemical (IHC) stains on biopsy specimen of mediastinal mass and TCR-beta/gamma gene rearrangement test were performed. Results: Among four patients, three were finally diagnosed as T-LBL, and one of them presented TCR beta and TCR gamma gene rearrangement on bone marrow. The other patient was diagnosed as thymoma proven by cytokeratin (AE1 and AE3) positivity of IHC stain in polygonal cells on biopsy specimen. All the patients presented immature T lymphoid cells with cCD3-positive and various expressions of the other markers in the pleural fluid. A patient with thymoma showed a smearing pattern of normally maturing T cells on CD3/CD45 plot (Fig 1A) while three with T-LBL showed a tight population or an atypical smearing pattern compared with normal maturation pattern (Fig 1B-D). One of three T-LBL patients showed a homogeneous population of double negative T cells on CD4/CD8 plot (Fig 1B). The other T-LBL patients showed a heterogeneous population with a majority of double positive T cells on CD4/CD8 plot and these double positive T cells showed aberrant expression of CD10 with varying intensities (Fig 1C, D). None of the T-LBL patients showed aberrant loss of other T cell markers including CD2, CD5, and CD7. Summary/Conclusion: In our study, the most useful marker for distinguishing T-LBL from normal thymocytes was a smearing pattern of normal maturation in the CD3/CD45 plot, followed by the aberrant expression of CD10 in double positive T cells. Although immunophenotypic distinction of T-LBL from thymocytes originated from thymoma remains still challenging in the pleural fluid, the immunophenotypic characteristics observed in MFC will provide helpful information when rapid diagnosis is required or when biopsy is difficult Fig. 1. Flow cytometry plot of pleural fluid. Arrow represents maturation curve of normal thymocytKeywords: Thymus, T cell lymphoma, T-ALL, Flow cytometry